Diffusion bonding utilizing transient liquid phase

ABSTRACT

Diffusion bonding is effected utilizing a thin alloy interlayer which melts at the desired diffusion bonding temperature forming a transient liquid phase and which subsequently resolidifies at temperature as a result of constituent interdiffusion, continued heat treatment providing a homogeneous solid-state diffusion bond.

United States Patent Paulonis et al.

1 1 July 25, 1972 Scott Duvall, Middletown; William A. Owczarski,Cheshire, all of Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[22] Filed: April 1, 1971 21 Appl. No.1 130,149

52 us. c1 ..29/498, 29/504 51 1111. c1. .B23k 31/02, 823k 35/24 58 Field6: Search ..29/49s, 504

[56] References Cited UNITED STATES PATENTS 2,957,239 10/1960 Pl'ilChard..29/49:; x 3,024,109 3/19 2 Hoppin et a1. ....29/49& x 3,088,192 5/1963Turner... 29/504 x 3,145,466 8/1964 Feduska .1 ...29/504 x 3,188,7326/1965 Feduska et al ..29/504 x O O O O 3,197,858 8/1965 Feduska et al.29/504 X 3,530,568 9/1970 Owczarski et al 29/498 3,552,898 l/l97l Birdet al ..29/498 OTHER PUBLlCATlONS Kaarlela et 01., Alloy Effects in theLow-Pressure Diffusion Bonding of Superalloys," Welding Journal, June,i967. pp. 283- s to 288- s.

Kinelski et al., New Developments in Brazing HighTemperature Nickel-BaseAlloys, Welding Journal, Dee. 1959, pp. 482-s to 486- s.

Primary Examiner-John F. Campbell Assistant Examiner-Ronald J. ShoreAnorney-Richard N. James [5 7] ABSTRACT Diffusion bonding is effectedutilizing a thin alloy interlayer which melts at the desired diffusionbonding temperature forming a transient liquid phase and whichsubsequently resolidifies at temperature as a result of constituentinterdiffusion, continued heat treatment providing a homogeneoussolid-state diffusion bond.

1 Claim, 7 Drawing Figures PATENTED JUL25 i972 SHEET 1 0F 2 DIFFUSIONBONDING UTILIZING TRANSIENT LIQUID PHASE BACKGROUND OF THE INVENTION Thepresent invention relates in general to the art of diffusion bonding,particularly as applied to the superalloys.

It is frequently desirable to make certain gas turbine engine componentsby joining easily fabricable segments together into the desiredconfigurations. However, the limited weldability of the high strengthsuperalloys has severely restricted the application of fusion weldingtechniques on structural turbine hardware. Further, many componentsbecause of configuration are simply not adapted to .fusion weldingtechniques. Brazing, while offering a number of advantages over fusionwelding, has very limited application because of the penaltiesassociated with the relatively low strengths and low melting pointsassociated with the typical brazed joints.

Diffusion bonding, which involves the solid-state movement of the atomsand grain growth across a joint interface, offers particular promise asthe joining technique for the highly alloyed superalloys. It has beendemonstrated, for example, that complex assemblies may be fabricatedfrom the superalloys by diffusion bonding with the provision of bondedareas which are practically indistinguishable from the adjacent parentmetal even on close metallurgical examination. In this regard, referencemay be made to the patent to Owczarski et al., U.S. Pat. No. 3,530,568.

Since diffusion bonding basically depends upon solid-state transportphenomena, it necessarily follows that carefully prepared and matchedsurfaces are mandatory for the achievement of satisfactory joints. Thisin turn makes the typical diffusion bonding fabrication process bothsensitive to precise process control and expensive. Furthermore, evenwith precise control of the process parameters, certain desirableinterface geometries are quite difficult to bond if pressure is notapplied uniformly throughout the entire interface area, and interfacialdefects often result if insufficient localized deformation cannot beeffected over the entire interface area. The problem is, of course,principally related to the necessity for sufficient surface-to-surfacecontact for the solid-state transport phenomena to occur uniformlyacross the joint.

Brazing operations in general are not as sensitive to a surface match asis diffusion bonding, inasmuch as the flow of the molten braze materialtends to fill any gaps existing between the surfaces. Furthermore, it iswell recognized that in all types of brazing a necessary measure ofinteralloying takes place between the braze material and the base metalsubstrate.

In a brazed joint, the efficiency is dependent not only upon theeffectiveness of thebraze/substrate bond but also upon the compositionof the braze material itself upon completion of the brazing cycle. Somebrazing operations permit sufficient interalloying of the braze materialwith the substrate to an extent providing a brazed joint having a remelttemperature in excess of the original brazing temperature. If anelement, such as boron, is included in the braze alloy the meltingtemperature of the braze material may be depressed. Subsequent rapiddiffusion of the boron into the substrate during the bonding operationeffectively raises the remelt temperature of the joint although it doesnot provide a homogeneous diffusion-bonded joint. Representative oftechniques of this nature is that described by Boam et al., US. Pat. No.2,714,760.

In subsequent developments, others have investigated bonding systemswherein brazing and diffusion bonding process features have beencombined. See, for example, an article by Davies et al in the BritishWelding Journal, Mar. 1962, entitled Diffusion Bonding and PressureBrazing of Nimonic 9O Nickel-Chromium-Cobalt Alloy. In thesedevelopments, the melting point depressant effect of boron, carbon orsilicon together with their high diffusion rates have been utilized asinterface materials in diffusion bonding processes, the interfacematerial, in essence, assuring initial good surface contact fordiffusion bonding through melting and flow in the early stage of thebonding sequence. Subsequent diffusion provides solid-state bonding.

While many of the braze alloy formulations utilized in these laterdevelopments have been of relatively simple chemistry and thus have notincorporated many of the strengthening mechanisms characterizing thesuperalloys, it has been reported that others have investigated combinedbrazing/diffusion bonding techniques utilizing interface alloysconsisting of the base metal alloy to be bonded simply doped with amelting point depressant such as boron, etc., sufficient in quantity toprovide melting at the desired temperature.

Although the art has thus suggested that a combined brazing/diffusionbonding process may be applied to various materials, application of thevarious techniques to the socalled rich superalloys is not asstraightforward as the art might appear to suggest; As illustrative ofthe problem is the fact that while the various melting point depressantssuggested in the art such as boron, silicon, manganese, columbium andtitanium, do in fact provide interlayer alloys with satisfactory meltingpoints, most are unusable with the nickelbase superalloys because of theproduction of deleterious phases during diffusion. In addition, problemssuch as lack of wettability. inability to control interlayer thickness,and severe base metal corrosion may be present.

SUMMARY OF THE INVENTION A diffusion bonding process is describedwherein a thin interlayer alloy of specific composition is placedbetween the surfaces to be bonded. The assembly with the surfaces heldtogether is heated to the bonding temperature (above about 1930F.) wherethe interlayer melts filling the gaps between the surfaces. While theassembly is held at temperature rapid interdiffusion occurs causing thejoint to isothermally solidify, creating an initial bond. Further timeat temperature or subsequent heat treatment is utilized to provide adiffusion bond and complete joint homogeneity and joint efiiciencies ofpercent or more.

The thin (typically 0.0005-0.005 inch) interlayer alloy is formulated toa chemistry substantially corresponding to that of the metal or metalsto be joined except that aluminum, titanium and carbon are excluded, andsufficient boron up to about 5 weight percent is present to depress themelting point ofthe interlayer alloy to a temperature at which the metalsubstrate can be exposed without deleterious effect.

BRIEFDESCRIPTION OF THE DRAWINGS FIGS. 15 depict the bonding sequenceutilized in the present invention.

In FIG. I the interlayer alloy containing readily difiusible speciessuch as boron, illustrated by solid circles, is sandwiched between thesurfaces to be joined.

FIG. 2 illustrates the assembly heated to the bonding tem' perature, theinterlayer materials having melted and flowed to fill the gaps betweenthe surfaces.

FIG. 3 illustrates diffusion of interlayer species into the surfaces tobe joined and isothermal solidification of the interlayer alloy.

FIG. 4 shows continued exposure at temperature and the continuation ofhomogenization.

FIG. 5 depicts the completed joint.

FIG. 6 is a graphic reproduction of electron-probe microanalysis tracestaken across an actual joint made as taught herein.

FIG. 7 demonstrates the results of 1,800 F. stress rupture testsindicating 95 percent joint efficiency for the present diffusion bondsmade by the present technique in a nickelbase superalloy system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred process of thisinvention (hereinafter referred to as the TLP Bonding Process)contemplates initially electroplating the surfaces to be joined with avery thin layer (0.0001 inch) of nickel or a nickel-cobalt alloy. Ifproper surface protection is otherwise provided to prevent contaminationof the faying surfaces, however, the initial electroplate may beeliminated. A thin (typically 0.0005-0.005 inch) interlayer alloy ofspecific composition, as hereinafter discussed in greater detail, isplaced between the mating surfaces (as in FIG. 1) and the surfaces areheld together. Only a slight positive pressure sufficient to hold theparts together (typically less than 10 p.s.i.) is required. Lesserinterlayer thickness may be utilized if the surfaces are very closelymatched.

The assembly is then heated in vacuum (usually l torr) to the desiredbonding temperature above the solvus temperature of the 7' phase in thenickel-base superalloy systems being bonded (typically in the range of2,000-2,200 F.) or close to the solvus for the very rich alloys where'y' solvus approaches the incipient melting point of the alloys and, inany event, above about l,930 F. At this temperature the interlayer alloymelts and a thin layer of liquid wets and fills the gaps between the twomating surfaces (FIG; 2). While the assembly is held at temperature,rapid diffusion of certain alloying elements occurs between theinterlayer and the base metal, resulting in a compositional change atthe joint. This change raises the local melting point and causes thejoint to isothermally solidify thus creating the initial bond (FIG. 3).Because the interlayer solidifies epitaxially and under dynamicequilibrium at the bonding temperature, it does not contain thedifficult-to-eli'minate multiple phase structure attendant with thenonequilibrium solidification characterizing fusion welding orconventional brazing. Instead upon completion of the initial isothermalsolidification (typically in 1-3 hours), the joint microstructureresembles that of the base metal except for some compositional andstructural heterogeneity (FIG. 4).

Ideally, the next step is continuation of the heat treatment attemperature for-a time sufficient to completely homogenize the jointregion so that, ultimately, it reaches a composition corresponding or atleast closely equivalent to the base metal, although a separate anddistinct subsequent heat treatment may be utilized. After completion ofthe bonding process, the bonded assembly can then be given whateverfurther heat treatments are required for strengthening or in fulfillmentof coating requirements. Preferably, when possible, the times andtemperatures utilized for homogenization of the joint area are selectedto also correspond with the high temperature heat treatments specifiedfor the alloy substrate.

It is evident that because minimal pressure is required during thejoining operation, parts of complex geometry can be TLP bonded withoutdeformation or other possible deleterious effect such as excessive basemetal grain growth. Further, special tooling is not required sinceuniform high interfacial pressures are not necessary, and mating surfacefinish and fixup requirements are also substantially relaxed. Finally,the TLP Bonding Process is suitable for the processing of large numbersof parts in conventional vacuum heat-treating furnaces.

The most critical features of the TLP Bonding Process are thecomposition. and characteristics of the interlayer alloy. It must meltat a temperature at which the base metal can be exposed withoutdeleterious effect but must be such that, in terms of composition andthickness, solidification will occur at temperature, and chemical andmicrostructural homogeneity may be achieved in a practical processingtime.

Various melting point depressants such as boron, silicon, manganese,columbium and titanium have been evaluated. Several combinations ofthese elements produceinterlayers with satisfactory melting points.However, with the rich nickel-base superalloys all except boron alsoproduce unwanted stable phases at the joint interface. Consequently,only boron is utilized in the TLP Bonding Process. The boron content iscontrolled to obtain an optimum balance between melting point and easeof subsequent homogenization. In terms of overall interlayer alloychemistry the alloy is otherwise formulated to closely conform inchemistry to the base metals being joined except that, because ofunfavorable phase formulation problems, aluminum, titanium and carbonare excluded.

These elements are, in fact, replenished in the interlayer region bydiffusion from the base metal during homogenization. The exclusion ofthese elements also appears to provide good wettability to thesubstrate, a prerequisite for the attainment of sound homogeneousbonding.

EXAMPLE A TLP bond was produced between two surfaces of a wroughtnickel-base alloy of the following nominal composition, by weight, 15percent chromium, 18.5 percent cobalt, 3.3 percent titanium, 4.3 percentaluminum, 5 percent molybdenum, 0.07 percent carbon, 0.03 percent boron,balance nickel (Alloy A).

The surfaces were first electroplated with a nickel flash to a thicknessof 0.000] inch. An interlayer alloy of the nominal composition, byweight, l5 percent chromium, 15 percent cobalt, 5 percent molybdenum, 3percent boron, balance nickel, in the form of 0,003 inch sheet wassandwiched between the plated surfaces and the assembly was bonded invacuum 10" torr) at 2, 1 40 F. for 1 hour under a dead weight pressureof about 8 p.s.i. Subsequently, the sample was homogenized by heattreatment in argon at 2,l40 F. for 24 hours and then the assembly wasgiven the standard aging heat treatment (1975 F./4 hrs. l550 F./4 hrs.1.400" F./l6 hrs.).

As may be seen from FIG. 5, electron probe microanalysis across thejoint area revealed no compositional differences between the bond regionand the base metal. Metallographic examination also revealed the jointarea to be free of porosity and extraneous phases and revealed aninterfacial grain boundary structure which is irregular and nonplanar.

Satisfactory bonds have also been made between two pieces of cast AlloyA utilizing the foregoing process parameters and interlayer alloy.Similarly, TLP bonds have been made between different nickel-basesuperalloys. In one test wrought Alloy A was joined to cast Alloy Bhaving a nominal composition of, by weight, 14 percent chromium, 4.5percent molybdenum, 2 percent columbium, 1 percent titanium, 6 percentaluminum, 0.0l percent boron, 0.08 percent zirconium, balance nickel.The same interlayer alloy used with wrought Alloy A wasv utilized andthe sample was both bonded and homogenized at 2,l40 F. which temperaturecorresponds to the solution heat treatment temperature for Alloy A.

From the foregoing description it will be evident that a relativelysimple way has been discovered to make efficient diffusion bonds,particularly bonds characterized by substantially complete jointhomogeneity and free of porosity and undesirable phases andmicrostructural defects. The invention in its broader aspects is not,however, limited to the specific details shown and described butdepartures may be made from such details without departing from theprinciples of the invention and without sacrificing its chiefadvantages.

What is claimed is:

I. A diffusion bonding process for the high strength nickelbasesuperalloys of the 'y 1 type which comprises:

providing an interlayer alloy having a composition substantiallycorresponding to that of the superalloys being joined, except asfollows, aluminum, titanium and carbon are substantially excluded, andboron in an amount up to about 5 weight percent is present as atemperature depressant, the interlayer alloy having a melting pointabove about l,930 F. and within the solvus temperature range of thesuperaly; sandwiching the interlayer alloy in a thickness of0.0005-0.005 inch between the surfaces to be bonded with sufficientforce to maintain the respective surfaces in intimate contact therewith;heating the sandwich assembly to a temperature above the melting pointof the interlayer alloy;

holding the assembly at temperature until isothermal solidification ofthe joint occurs by difiusion; and diffusion heat treating the assemblyto provide homogenization of the joint area.

